Aluminum Alloy For Die Casting, And A Method For Producing A Cast Product Using The Aluminum Alloy

ABSTRACT

An aluminum alloy suitable for obtaining a die-cast aluminum product having high strength and high toughness, comprises, by mass, Si: 7.0 to 9.0%, Mg: 0.4 to 0.6%, Cu: 0.4 to 0.7%, Cr: 0.5% or less, Mn: 0.5% or less, [Cr+Mn]: 0.1 to 0.8%, Fe: 0.10 to 0.25%, and Sr: 0.005 to 0.02%, the balance being Al and impurities.

CROSS REFERENCE TO RELATED APPLICATION

This application is based upon and claims the benefit of priority to Japanese Patent Application No. 2019-136522 filed on Jul. 25, 2019, the entire contents of which are incorporated herein by reference.

BACKGROUND

The present disclosure relates to an aluminum alloy suitable for die casting by a die casting machine, and a method for producing a cast product by die casting using the aluminum alloy, and in particular, the disclosure can achieve a balance between high strength and high toughness.

Die casting is a casting process for filling molten metal in the cavity of a metal mold at a high speed under a high pressure, is of high productivity, and has been widely employed in automobile parts, machinery parts, and the like.

As aluminum alloys for use in such a die casting, Al-Si based alloys are used because of requirement of fluidity in the metal mold; however, in recent years, an aluminum alloy having higher strength have been required from a standpoint of needs for weight reduction, but such aluminum alloys tend to have insufficient toughness, and therefore, expectations have been placed on an aluminum alloy having high strength and high toughness.

Japanese Patent No. 5575028 discloses an aluminum alloy containing Si: 3.5 to 7.5%, Mg: 0.45 to 0.8%, Cr: 0.05 to 0.25%, and Cu: 0 to 0.01%, and states that solution heat treatment and subsequent heat treatment that is so-called T6 treatment are necessary to achieve tensile strength of 320 MPa or more.

However, the process step of T6 treatment takes a longer period of time than the process step of T5 treatment, and therefore, the productivity of T6 treatment is inferior to that of T5 treatment.

In Japanese Patent No. 5575028, Cu as a constituent is as low as 0 to 0.01%, and it is therefore estimated that sufficient strength is not achieved by T5 treatment.

Japanese Patent No. 5797360 discloses an alloy containing Si: 6.00 to 6.50%, Mg: 0.10 to 0.50%, Fe: 0.30% or less, Mn: 0.30 to 0.60%, and Cr: 0.10 to 0.30% for the purpose of achieving high proof stress and high ductility, and states that the alloy further contains Sr: 30 to 200 ppm, B: 1 to 50 ppm, Sb: 0.05 to 0.20%, and Ti: 0.05 to 0.30%; however, the fluidity is likely to be low because of low content of Si, and in addition, Cu as a constituent is added in a slight amount, and it is therefore estimated that sufficient strength is not achieved by T5 treatment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates the chemical composition of the aluminum alloys used in the evaluation;

FIG. 2 illustrates the evaluation results of the cast products; and

FIG. 3 illustrates optical microscope photographs of metal structures in Example 1 and Comparative Example 1.

DESCRIPTION OF EXEMPLARY EMBODIMENTS

The following disclosure provides many different embodiments, or examples, for implementing different features of the provided subject matter. These are, of course, merely examples and are not intended to be limiting. In addition, the disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. Further, when a first element is described as being “connected” or “coupled” to a second element, such description includes embodiments in which the first and second elements are directly connected or coupled to each other, and also includes embodiments in which the first and second elements are indirectly connected or coupled to each other with one or more other intervening elements in between.

It is an object of the disclosure to provide an aluminum alloy suitable for obtaining a die-cast aluminum product having high strength and high toughness, and a method for producing a cast product using the aluminum alloy.

In accordance with one of some embodiments, there is provided an aluminum alloy for die casting, comprising, by mass, Si: 7.0 to 9.0%, Mg: 0.4 to 0.6%, Cu: 0.4 to 0.7%, Cr: 0.5% or less, Mn: 0.5% or less, ([Cr+Mn]: 0.1 to 0.8%, Fe: 0.10 to 0.25%, and Sr: 0.005 to 0.02%, the balance being Al and impurities.

It is also possible that only one of Cr and Mn are contained alone, and the content of the total of Cr and Mn, [Cr+Mn], is set in a range of 0.1 to 0.8%.

In the disclosure, impurities refer to inevitable impurities that may be unintentionally incorporated during the production step.

Furthermore, if needed, Ti may be contained in an amount of 0.2% or less.

The reason why the ranges of the contents of the constituents are specified in the disclosure will be described.

Si as a Constituent

Si is a constituent of importance for pouring the molten metal of the aluminum alloy into cavity in the die casting, and in order to ensure such fluidity, 7.0% by mass (hereinafter, simply expressed as %) or more of Si is necessary.

However, in a case where the content of Si is greater than 9.0%, when the molten metal is cooled in the metal mold, a coarse pro-eutectic Si tends to precipitate, and the elongation may be decreased, and therefore, the content of Si is set in a range of Si: 7.0 to 9.0%.

Mg and Cu as Constituents

Mg and Cu are effective constituents for enhancing the strength of the cast product, and in the disclosure, the contents of Mg and Cu are set at Mg: 0.4% or more and Cu: 0.4% or more in order to achieve high strength by heat treatment (T5 treatment) after the casting.

In this regard, for the purpose of suppressing excessive decrease in the elongation, the contents of Mg and Cu are set at Mg: 0.4 to 0.6% and Cu: 0.4 to 0.7%.

Cr, Mn, and Fe as Constituents

In die casting, the metal mold, the machine, and the like are made of iron based alloys, and therefore, Fe as a constituent is one of constituents that tends to be unintentionally incorporated. When the content of Fe is greater than 0.25%, the crystallization product is coarsened, and the elongation is decreased, and therefore, in the disclosure, the molten metal of the aluminum alloy is controlled in order to suppress the content of Fe to be in a range of Fe: 0.10 to 0.25%.

Addition of Cr or Mn allows for refining the above Fe-based crystallization product crystalized when the molten metal of aluminum alloy is cooled in the metal mold, and in a case where Cr or Mn is added alone, the content of Cr may be 0.5% or less, preferably in a range of 0.1 to 0.5%, or the content of Mn may be 0.5% or less, preferably in a range of 0.1 to 0.5%.

In this regard, the content of the total of Cr and Mn, [Cr+Mn], is preferably 0.1 to 0.8%.

When the content of [Cr+Mn] is greater than 0.8%, a coarse crystallization product of Cr·Mn may be crystalized.

In the disclosure, both of Cr and Mn can be contained; and it is further preferable that both of Cr and Mn are contained, and the content of [Cr+Mn] is preferably in a range of [Cr+Mn]: 0.3 to 0.8%.

Sr and Ti as Constituents

Addition of Sr in a small amount is effective for refining of eutectic Si, and the content of Sr is set in a range of Sr: 0.005 to 0.02%.

Although Ti is not an essential constituent in the disclosure, addition of Ti is effective for refining of crystal grains in the aluminum alloy, and when Ti is added, the content of Ti is 0.2% or less, preferably in a range of 0.01 to 0.2%.

The metal mold used in die casting consists of a fixed mold and a movable mold that is controlled to open or close with respect to the fixed mold, and a cavity is formed in the closed metal mold. A molten metal of an aluminum alloy introduced in a sleeve is injected into the cavity under the condition of a high pressure with a plunger for producing a cast product.

Such injection conditions are important for uniform filling the molten metal of the aluminum alloy in the cavity of the metal mold, and higher filling speed leads to higher productivity; however, when turbulent flow is generated in a flow of molten metal, air tends to be unintentionally mixed in casting.

Therefore, it is preferable that an injection gate speed is set at 1 m/sec or less so that the aluminum alloy is filled in a laminar flow into a cavity of the metal mold through a gate of the metal mold. The injection gate speed means a filling speed of the molten metal at the gate which communicates to the cavity. The smaller the gate, the faster the molten metal will flow into the cavity, creating a turbulent flow and more air entrainment. The present invention enlarges the gate to prevent the melt from becoming turbulent.

As the metal mold used in the disclosure, it is preferable to use a metal mold of center gate type that allows for the molten metal to be injected radially from the center of the metal mold so that the flow of the molten metal in the cavity is uniform.

By means of using the aluminum alloy according to the disclosure, it is possible to obtain a cast product having high strength and high toughness with tensile strength of 280 N/mm² or more, a 0.2% proof stress of 180 N/mm² or more, elongation of 4% or more, and a Charpy impact value of 3 J/cm² or more by performing, after the die casting, heat treatment (T5) at 160° C. to 200° C. for 2 to 8 hours.

In the metal texture of such a cast product, the average area of the crystallization product excluding eutectic Si is 2 μm² or less as measured by optical microscopy.

When die casting is performed using the aluminum alloy according to the disclosure, a die-cast product having high strength and excellent toughness can be obtained by T5 treatment after the die casting.

Exemplary embodiments are described below. Note that the following exemplary embodiments do not in any way limit the scope of the content defined by the claims laid out herein. Note also that all of the elements described in the present embodiment should not necessarily be taken as essential elements

A molten metal of the aluminum alloy having each chemical composition illustrated in FIG. 1 is adjusted, and die-cast using a metal mold of center gate type to obtain a cast product.

The casting conditions are the molten metal temperature of about 700° C., and an injection gate speed of 0.8 m/sec.

After the casting, heat treatment (T5) is performed at 190° C. for 3 hours.

FIG. 2 illustrates the measurement results regarding the mechanical properties of the cast products, and the average areas of the crystallization products excluding eutectic Si (average areas of crystallization products).

FIG. 3 illustrates examples of optical microscope photographs of Example 1 and Comparative Example 1.

The test items and test method are as follows.

Tensile Strength, 0.2% Proof Stress, and Elongation

In accordance with JIS-Z2241, a JIS-4 test piece is cut from a cast product, and a tensile test is performed by using a tensile tester according to Japanese Industrial Standards to determine the tensile strength (MPa), the 0.2% proof stress (MPa), and the elongation (%).

Charpy Impact Test

In accordance with JIS-Z2242, a JIS-4 Charpy impact test piece is cut from a cast product, and is subjected to the measurement by using a Charpy impact tester according to Japanese Industrial Standards.

Average Area of Crystallization Product

The cut surface of the cast product is mirror-polished, and subjected to observation and measurement by using an optical microscope.

Image processing is performed in the range of measurement area of 0.166 mm² at a magnification of 1000 times, and areas of the crystallization products in the portions excluding eutectic Si are measured to obtain the average value.

The following facts can be seen from the chemical composition in FIG. 1 and the evaluation results in FIG. 2.

In the disclosure, high strength with tensile strength of 280 MPa or more, a 0.2% proof stress of 180 MPa or more is considered to as a goal, and also, high toughness with elongation of 4% or more and a Charpy impact value of 3 J/cm² or more is considered to as a goal.

Any of Examples 1 to 13 has an alloy composition in the set range, and all the goals are satisfied by any of the cast products which are removed from the metal mold after the casting and to which the T5 heat treatment is performed at a relatively low temperature of 160 to 200° C.

For the purpose of achieving high strength after the die casting, as described in Japanese Patent No. 5575028, so-called T6 treatment has been conventionally performed, in which, in order to perform solution treatment of precipitated constituents that precipitate when the product is heated to a high temperature of 500° C. or more and cooled in the metal mold, the liquefaction treatment is performed, and subsequently, quenching is performed by water cooling or oil cooling, followed by heat treatment.

In this case, distortion may be created in the product in the rapid cooling.

In addition, although the strength is high, the toughness is decreased.

In Comparative Example 1, the content of Cr is 0.85%, and is greater than Cr: 0.5%, and as a result, a coarse Cr-based crystallization product is crystalized, and the elongation (3%) is less than the goal (4% or more), and also, the Charpy impact value (2.5) does not reach the goal (3 or more).

In Comparative Example 2, either Cr or Mn is not added, the total amount of [Cr+Mn] is less than 0.1%, and as a result, the Fe-based crystallization product is coarsened, and the value of the elongation is low.

In Comparative Example 3, the amount of Cu is less than the set range, in Comparative Example 4, the amount of Si is less than the set range, and in Comparative Example 5, the amount of Mg is less than the set range, and as a result, the tensile strength and the 0.2% proof stress do not reach the goals.

In Comparative Example 6, the amount of Cu is larger than the set range, in Comparative Example 7, the amount of Mg is larger than the set range, and in Comparative Example 8, the amount of Fe is larger than the set range, and as a result, the elongation is low.

In Comparative Example 9, Sr is not added, and as a result, the elongation is degraded.

Comparative Examples 10 to 14 are of as-fabricated, and as a result, the strength is low.

In Comparative Example 15, the total value of [Cr+Mn] is 0.98%, and is greater than 0.8%, and as a result of this, the crystallization product is coarsened, the average area of the crystallization product is 5.6 μm², and is greater than 2 μm², and the elongation is low.

In Comparative Example 16, Mn is 0.72%, and is greater than 0.5%, and as a result, the elongation is low.

In the chemical compositions illustrated in FIG. 1, Zn, Ni and Sn are inevitable impurities, and are preferably in an amount as small as possible, and Zn, Ni and Sn may be contained in amounts of 0.05% or less.

In die casting, a molten metal obtained by placing an aluminum alloy in a furnace, and heating and melting the aluminum alloy is injected in a metal mold at a high speed under a high pressure, and as a result, the aluminum alloy is rapidly cooled in the metal mold, and solidifies.

Therefore, in order to distribute the molten metal to every portions of the cavity in the metal mold, Si is added to ensure the fluidity, and in addition, a crystallization product of Si is precipitated in the cooling process of the casting, resulting in the enhancement of the strength; however, there is a problem in that, when the amount of Si added is large, the precipitate is coarsened, and the toughness is decreased.

Therefore, in the disclosure, the content of Si is set in a range of Si: 7.0 to 9.0%; however, for further improvement in the fluidity in casting, it is preferable that the content of Si is set at Si: 8.0 to 9.0%.

However, the toughness is decreased without refining of the size of the precipitate.

In the disclosure a trace amount of Sr is added to refine the size of eutectic Si for the purpose of suppressing decrease in the toughness.

When the content of Si is in a range of Si: 7.0 to 9.0%, the effect resulted from addition of Sr can be identified, and particularly in this range, when the content of Si is Si: 8.0 to 9.0%, addition of Sr is essential.

Because an iron based metal mold, iron based machine, and the like are used in die casting of an aluminum alloy, there is a limit of suppressing contamination of Fe as a constituent on the course of repeatedly casting the aluminum alloy maintained to be in a molten state.

Therefore, in the disclosure, the Fe-based crystallization product is refined by addition of one or both of Cr and Mn.

In particular, addition of Mn in a small amount of 0.1 to 0.3% suppresses seizure of a product to the metal mold in casting.

Although only some embodiments of the present disclosure have been described in detail above, those skilled in the art will readily appreciate that many modifications are possible in the embodiments without materially departing from the novel teachings and advantages of this disclosure. Accordingly, all such modifications are intended to be included within scope of this disclosure. 

What is claimed is:
 1. An aluminum alloy for die casting, comprising, by mass, Si: 7.0 to 9.0%, Mg: 0.4 to 0.6%, Cu: 0.4 to 0.7%, Cr: 0.5% or less, Mn: 0.5% or less, [Cr+Mn]: 0.1 to 0.8%, Fe: 0.10 to 0.25%, and Sr: 0.005 to 0.02%, the balance being Al and impurities.
 2. The aluminum alloy for die casting according to claim 1, further comprising 0.2% or less of Ti.
 3. A method for producing a cast product, comprising die-casting the aluminum alloy according to claim 1, and filling the aluminum alloy in a laminar flow into a metal mold at an injection gate speed of 1 m/sec or less.
 4. The method for producing a cast product according to claim 3, wherein heat treatment is performed at 160° C. to 200° C. for 2 to 8 hours after the die casting.
 5. The method for producing a cast product according to claim 4, wherein the cast product has high strength and high toughness with tensile strength of 280 N/mm² or more, a 0.2% proof stress of 180 N/mm² or more, elongation of 4% or more, and a Charpy impact value of 3 J/cm² or more. 